A magnetic compass is an instrument containing a freely suspended magnetic element which displays the direction of the horizontal component of the Earth's magnetic field at the point of observation. The principle concept of a magnetic compass dates back to the Chinese Qin Dynasty (221–206 B.C.) when fortunetellers observed lodestones (iron oxide) would align themselves in a north/south orientation. This phenomenon was later employed to determine orientation with respect to cardinal points and stellar constellations. In later years, compasses proved to be valuable navigational aids for ocean-going ships. In modern times, even with the advent of precise navigational instruments, such as a Global Positioning System (GPS), the magnetic compass remains an invaluable tool. Whether military, explorer, or recreational outdoorsman, the simple magnetic compass provides direction and comfort to a person in an unfamiliar territory.
Magnetically sensitive instruments, such as a compass, must be housed or mounted in such a way as to prevent magnetically active components, for example screws or other metallic components, from interfering with the sensitive compass needle. The amount of magnetic interference generally increases with the size and proximity of a metallic part to the compass needle. This problem has largely been resolved through the use of plastics as housing for magnetically sensitive instruments. For example, compasses can be housed entirely within a plastic case and either mounted a distance away from any metallic object, or in the case of a hand-held compass, be held away from metallic objects.
Often one wishes to provide light to enhance the readability of a magnetic compass in low light conditions. A hand held compass is often used in dense forests or jungles, or in caves where ambient light, particularly at night, is not sufficient to see the needle. There have been many attempts in the prior art to provide illumination for a compass. However, if any electronic components, such as incandescent bulbs, are incorporated into the instrument, their magnetic field will interfere with the instrument's reading.
U.S. Pat. No. 6,145,209 relates to an illumination device adapted to be mounted on a transparent bottom of a magnetic compass for illuminating a compass rose and a compass needle thereof. The light source comprises an incandescent lamp or LED affixed to a mounting plate. When the light source is activated, the lamp projects light upward through the bottom of a transparent case containing the compass rose.
U.S. Pat. No. 6,578,277 relates to a magnetic compass comprising a compass body, an illuminant, a battery set, a base, and a top lid. The compass body, the illuminant and the battery set are situated within the interior section of the base and, furthermore, the illuminant is positioned at one side of the compass body and its light is projected towards the peripheral walls of the compass body. As such, the compass body is illuminated and the magnetic compass of the present invention is effectively usable under low light conditions.
Conventional lighting sources and the electrical wiring associated with them are known to radiate electromagnetic (EM) energy. This EM energy field will affect the operation and orientation of a compass needle, thereby providing an inaccurate directional heading. Incandescent bulbs and un-insulated copper wire are notorious for emitting EM fields. Generally, the interference caused by an EM energy field radiated from a small incandescent light only produces small perturbations in the compass reading. Either of the two aforementioned compass illumination schemes may be employed in recreational compasses with a relative loss of compass accuracy which does not materially affect the casual user.
However, for certain applications, a higher degree of accuracy in compass readings is required and the interference from incandescent bulbs and wiring becomes a significant factor in compass error. While not limited to such uses, particular applications which require a high degree of compass accuracy comprise long range military operations, and outdoor adventurers. An error of a few seconds of a degree, over tens of miles, can result in significantly missing a target location.
These sophisticated compass users are also more likely than a recreational user to navigate at night or in low light conditions. One particular application of such a compass finds widespread utility in military operations where a night-readable compass is required to navigate. However flashlights and other bright, harsh, illumination sources are discouraged so as not to alert opposing forces to the presence of military personnel.
Therefore, there is a need to provide illumination to a magnetic compass or other magnetically sensitive instrument without interfering with the operation of the instrument. More particularly, there is a need to provide a source of illumination that produces little or no electromagnetic radiation. It is to this problem that the present invention is directed.